Bulk micromachining of silicon wafers is well known in the semiconductor arts. Generally, this process involves forming semiconductor devices on a silicon wafer by etching the bulk silicon at the surface of the wafer, in contrast to etching methods in which semiconductor devices are formed by selectively etching layers which were previously deposited on the surface of the wafer substrate. Bulk micromachining can be used to form micromachined features in the surface of a silicon substrate from which sensing devices can be formed, and is generally preferred over etching deposited layers in the fabrication of sensing devices in that less warpage occurs, thereby enhancing the accuracy of the sensing device. Bulk micromachining is often conducted using a conventional wet etch process, which is isotropic in nature. Dry etching processes, such as plasma etching, are becoming more common because of their capability for higher packing density as a result of being anisotropic in nature.
In the past, sensing devices have often been fabricated by stacking silicon wafers on top of each other so as to form a cavity over which a sensing micromachined element, such as a beam, bridge or membrane, can be formed with the upper wafer. Alignment tolerances as well as sharp corners and edges on the wafers create points of stress concentration within the sensing device, which interfere with the ability of the micromachined element to accurately detect the pressure or motion for which the device is intended. Consequently, bulk micromachining methods are often preferred in that the residual stresses and stress concentrators common to stacked-wafer techniques can generally be avoided.
A recent example of such a bulk micromachining method is disclosed by Zhang and McDonald (Digest IEEE Int. Conf. on Solid State Sensors and Actuators, pp. 520-523 (1991)), as generally illustrated in FIGS. 1a through 1f. Zhang and McDonald teach thermally depositing a silicon dioxide layer 102 on an arsenic-doped n-type &lt;100&gt; substrate 100 which is to be bulk micromachined. The silicon dioxide layer 102 is then photolithographically patterned using photoresist 104 which has been spun on the silicon dioxide layer 102, as indicated in FIG. 1a. A plasma etching process is then used to form trenches 106 to a depth of about 4 micrometers in the substrate 100, as shown in FIG. 1b.
A second layer of silicon dioxide (not shown) is then thermally grown on all exposed surfaces, followed by the deposition of another layer of silicon dioxide 108 using plasma enhanced chemical vapor deposition (PECVD), shown in FIG. 1c. After patterning and etching through the layers of silicon dioxide to provide a metal-to-substrate contact window, a layer of aluminum 110 is deposited on the upper layer of silicon dioxide 108, as indicated in FIG. 1d, from which electrodes are patterned. An anisotropic etch is then used to remove the silicon oxide 108 from the bottom of the trenches 106, as shown in FIG. 1e, and then an isotropic plasma etch is used to undercut the substrate 100 between the trenches 106 so as to form a cavity 114 beneath the surface of the substrate 100. As shown in FIG. 1f, the cavity 114 creates a suspended beam 112 which is suitable for sensing motion.
The above process is likely to be suitable for many applications, in that plasma etching techniques are capable of micromachining small features which can be integrated onto a chip containing integrated circuitry. However, the plasma etch process taught by Zhang and McDonald does not readily lend itself to forming selectively shaped cavities, in that the isotropic nature of the plasma etch requires that the process include a silicon dioxide deposition and etch to limit the direction of the etching action. Where no silicon dioxide layer or metal layer is present, the plasma etch will proceed uninhibited until the etching process is discontinued, as suggested by the shape of the cavity 114 shown in FIG. 1f. Accordingly, the process taught by Zhang and McDonald requires an oxide deposition and etch after the trench has been etched to roughly define the cavity. While such additional steps are entirely conventional, it is a continuous objective in the semiconductor industry to minimize the number of processing steps necessary to form any given device.
Furthermore, the teachings of Zhang and McDonald are limited to the formation of bridges and cantilevered beams. In other words, their teachings are absent any method by which larger structures can be formed, such as a suspended mass for motion sensing. Nor do their teachings suggest how the trenches can be suitably sealed so as to form membranes for sensing pressure or, alternatively, encapsulated so as to protect the bridge and cantilevered beams.
Thus, it would be desirable to provide an improved method for forming small, integrated micromachined elements in a silicon wafer using a bulk micromachining process, in which the method reduces the number of processing steps necessary to form the desired micromachined elements. Furthermore, it would be desirable that such a method be conducive to further processing by which the micromachined elements can be adapted to form various types of sensing devices having a wide variety of possible configurations.